Seamless looping animation
Things like fire etc. I loop in post-production, turns out to be the easiest way and fastest. Just render a bigger chunk of the animation, split it in half and switch the halves. The ends are now seamless but in middle is a discontinuity. Overlap those two parts and blend them with opacity and by pixel-warping if needed (visible edges should be pixel-warped). Picking the right moment for blending is crucial.

Reversing frames and having a ping-pong animation will not work on flags, fire, etc. - That's why it needs to be blended.